This invention relates generally to voltage-to-current converters. Specifically, the present invention relates to voltage-to-current converters which produce an output current that is linearly related to an input voltage. Moreover, the present invention relates to voltage-to-current converters which are entirely biased from a single power supply.
Prior art circuits fail to provide a common solution to three problems associated with voltage-to-current converters. One problem concerns achieving a linear conversion between a voltage input and a current output. A second problem concerns providing a circuit that operates from only one power source. Providing a converter that can operate over a wide range of input voltages presents the third problem.
Some prior art circuits which provide a linear conversion of voltage to current achieve linearity at a cost of requiring extra power sources or limiting the input voltage range over which the circuit operates. An input voltage is typically applied to one node of a resistor while the other node is connected to a reference potential. Thus, the input voltage is translated into an input current through the resistor and then converted into an output current by other circuitry. If ground represents this reference potential, then a linear conversion is possible. However, an active device such as an operational amplifier or transistor which measures the current in the resistor must be biased from two power sources, rather than a single power source, in order to measure current at a group potential.
Prior art linear voltage-to-current converters may operate from a single power source. These converters typically connect the resistor mentioned above to a reference potential which is above ground and provide additional circuitry which compensates for a resulting non-linearity. The non-linearity occurs because the current through the resistor reflects the difference between the input voltage and the reference potential divided by the resistance of the resistor, rather than the input voltage divided by the resistance of the resistor. Prior art circuits which use this approach limit the range of input voltages over which the circuit will operate because input voltages less than the reference potential cannot be converted. For example, the reference potential in a circuit shown in FIG. 2 of U.S. Pat. No. 4,443,753 entitled "Second Order Temperature Compensated Band Gap Voltage Reference" by Gerald F. McGlinchey, and issued Apr. 17, 1984, represents a voltage that is two diode drops above ground. Such a circuit could not convert input voltages less than the two diode-drops, or approximately 1.4 volts using the typical 0.7 volt diode-drop for silicon devices.
Additionally, prior art single power source, linear, voltage-to-current converters tend to sacrifice some degree of linearity in order to achieve the two diode-drop reference potential. Some circuits attempt to match the voltage drop which appears across the base and emitter nodes of an NPN transistor with the voltage drop which appears across the base and emitter nodes of a PNP transistor. However, the base-emitter voltages of NPN and PNP transistors cannot be precisely matched and can vary up to 0.100 volts or 15% from each other. Accordingly, matching NPN and PNP transistors results in a degradation of linearity in the voltage-to-current converter.
Accordingly, it is an object of the present invention to provide an improved voltage-to-current circuit which may operate entirely from a single power source.
Another object of the present invention relates to providing an accurate linear conversion of input voltage into output current by accurately compensating for any non-linearity which may be a consequence of using only one power source.
Still another object concerns accommodating a wide range of input voltages. The present invention converts input voltages as near to ground as the voltage represented by one diode-drop. Additionally, the present invention converts input voltages significantly greater than the voltage exhibited by the power source.
Yet another object relates to providing an improved voltage-to-current converter which exhibits an adjustable temperature coefficient that may be used to counteract detrimental temperature characteristics of other parts of a system. Alternatively, the temperature coefficient may be adjusted so that it is very low and the voltage-to-current conversion is substantially independent of temperature.
Another object requires the present invention to consume only a minimum amount of power during standby.